Volume Holograms for Security Documents

ABSTRACT

A method for producing a security document comprising a volume hologram, including the steps of: applying an optically sensitive material onto a first surface of a substrate in a first region of the first surface; and irradiating the optically sensitive material with patterned radiation configured for recording a volume hologram within the optically sensitive material.

FIELD OF THE INVENTION

The invention relates generally to optical devices for documents, forexample banknotes, such optical devices providing a security effect inthe form of a volume hologram visual effect.

BACKGROUND TO THE INVENTION

The use of holograms as a security device is well established in thefield of security documents, such as banknotes, passports, cheques,share certificates and the like. Holograms are, potentially, lesschallenging to counterfeiters to replicate because they are, relatively,well known and, relatively, well understood.

In order to reduce the risk of counterfeiting, many advances have beenmade in the field of holograms. However, these technological advancesare, generally, in the area relating to thin holograms. Where thickholograms (commonly known as volume holograms) have been investigated,it has been in relation to foils for attachment, for example by hotstamping, to a security document.

The process of hot stamping foils onto security documents has severaldisadvantages. For one, the foil can be prone to removal from thesecurity document, which can aid in replication and furthermore whichcan prematurely end the life of the security document (which is ofparticular importance in the field of banknotes). Another issue is thatfoil design can limit the range of different designs employable with thebanknote. Another disadvantage is that multiple steps are requiredcorresponding to forming a hologram on the foil and stamping the foilonto the security document.

Some applications of security documents require a minimal thickness ofapplied features. The combined thickness of a foil and a volume hologramcan in many instances result in a security device with a combinedthickness greater than that suitable for use with the security document.

SUMMARY OF THE INVENTION

It would therefore be desirable to provide for the production of volumehologram security devices which overcome the above noted problems.

According to a first aspect of the present invention, there is provideda method for producing a security document comprising a volume hologram,including the steps of: applying an optically sensitive material onto afirst surface of a substrate in a first region of the first surface; andirradiating the optically sensitive material with patterned radiationconfigured for recording a volume hologram within the opticallysensitive material.

Preferably, the optically sensitive material is applied using a printingand/or embossing process. Printing and/or embossing allows for variousadvantages, for example registration with other printed and/or embossedfeatures of the security document, and/or the creation of complexshapes.

In an embodiment, the method includes the further step of applying aprotective coating to an outwards facing surface of the opticallysensitive material. The protective coating may be applied after theirradiation of the optically sensitive material with patternedradiation.

The substrate is preferably formed from polymeric material, preferablybiaxially oriented polypropylene (BOPP). Also preferably, the opticallysensitive material is a printable photopolymer.

Optionally, once applied, the optically sensitive material includes asubstantially flat outward facing surface. The optically sensitivematerial is preferably not greater than about 20 microns thick, morepreferably not greater than about 10 microns thick.

In an alternative option, once applied, the optically sensitive materialincludes a non-flat outward facing surface, such that the opticalsensitive material provides a further optical effect in addition toproviding a volume hologram. Preferably, the outward facing surface isshaped as an arrangement of microlenses or is shaped as a diffractiongrating. Advantageously, this option allows for the combination of knownoptical effects typically created using radiation curable inks with thevolume hologram effect recorded into the optically sensitive material.This option may be particularly applicable to embodiments utilising aphotopolymer as the optically sensitive material.

The minimum thickness of the optically sensitive material is preferablysufficient to record a volume hologram.

In an embodiment, the method includes the step of: applying a firstopacifying layer to the first surface, the first opacifying layer beingomitted in a first window region, wherein the optically sensitivematerial is located within the first window region. The area of thefirst window region may be greater than, equal to, or less than the areaof the optically sensitive material. Preferably, the first opacifyinglayer and the optically sensitive material are applied in registration,such that the first window region is registered with the opticallysensitive material. The embodiment may include the step of applying asecond opacifying layer to a second surface of the substrate. The secondopacifying layer may be configured to partially or entirely cover theoptically sensitive material, such that where the second opacifyinglayer covers the optically sensitive material, the volume hologram isonly visible from the first surface. Alternatively, the secondopacifying layer may be omitted in a second window region, wherein theoptically sensitive material is visible through the second window regionof the second opacifying layer. The, or each, opacifying layer ispreferably applied after the irradiation with patterned radiation.

Optionally, a primer layer is applied to the first surface such that theoptically sensitive material is applied to the primer layer. Onepreferred primer material is polyethylene imine.

Optionally, the patterned radiation is provided by one or more lasers.The patterned radiation may be created in part through reflection oflaser radiation off an object and/or the patterned radiation may becreated in part through the provision of a master hologram. Preferably,the patterned radiation includes wavelengths selected from one or moreof the visible spectrum, infra-red spectrum, and ultra-violet spectrum.

According to another aspect of the present invention, there is provideda security document including a substrate having a first opacifyinglayer applied to a first surface, the first opacifying layer including afirst window region in which a volume hologram is located, said volumehologram formed from an optically sensitive material applied to thefirst surface.

The optically sensitive material may be applied directly to the firstsurface. Alternatively, the substrate may include a primer layer, andthe optically sensitive material is applied directly to the primerlayer.

Optionally, the substrate has a second opacifying layer applied to asecond side. The second layer may partially or entirely cover the volumehologram. Alternatively, the second layer may include a second windowregion substantially in register with the first window region in whichthe volume hologram is located, such that the volume hologram is locatedin a window of the security document.

According to another aspect of the invention, there is provided asecurity document produced according to the method of the first aspect.

A security documents according to an aspect of the invention ispreferably a banknote.

Security Document or Token

As used herein the term security documents and tokens includes all typesof documents and tokens of value and identification documents including,but not limited to the following: items of currency such as banknotesand coins, credit cards, cheques, passports, identity cards, securitiesand share certificates, driver's licenses, deeds of title, traveldocuments such as airline and train tickets, entrance cards and tickets,birth, death and marriage certificates, and academic transcripts.

The invention is particularly, but not exclusively, applicable tosecurity documents or tokens such as banknotes or identificationdocuments such as identity cards or passports formed from a substrate towhich one or more layers of printing are applied. The devices describedherein may also have application in other products, such as packaging.

Security Device or Feature

As used herein the term security device or feature includes any one of alarge number of security devices, elements or features intended toprotect the security document or token from counterfeiting, copying,alteration or tampering. Security devices or features may be provided inor on the substrate of the security document or in or on one or morelayers applied to the base substrate, and may take a wide variety offorms, such as security threads embedded in layers of the securitydocument; security inks such as fluorescent, luminescent andphosphorescent inks, metallic inks, iridescent inks, photochromic,thermochromic, hydrochromic or piezochromic inks; printed and embossedfeatures, including relief structures; interference layers; liquidcrystal devices; lenses and lenticular structures; optically variabledevices (OVDs) such as diffractive devices including diffractiongratings, holograms and diffractive optical elements (DOEs).

Substrate

As used herein, the term substrate refers to the base material fromwhich the security document or token is formed. The base material may bepaper or other fibrous material such as cellulose; a plastic orpolymeric material including but not limited to polypropylene (PP),polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC),polyethylene terephthalate (PET); or a composite material of two or morematerials, such as a laminate of paper and at least one plasticmaterial, or of two or more polymeric materials.

Transparent Windows and Half Windows

As used herein the term window refers to a transparent or translucentarea in the security document compared to the substantially opaqueregion to which printing is applied. The window may be fully transparentso that it allows the transmission of light substantially unaffected, orit may be partly transparent or translucent partially allowing thetransmission of light but without allowing objects to be seen clearlythrough the window area.

A window area may be formed in a polymeric security document which hasat least one layer of transparent polymeric material and one or moreopacifying layers applied to at least one side of a transparentpolymeric substrate, by omitting at least one opacifying layer in theregion forming the window area. If opacifying layers are applied to bothsides of a transparent substrate a fully transparent window may beformed by omitting the opacifying layers on both sides of thetransparent substrate in the window area.

A partly transparent or translucent area, hereinafter referred to as a“half-window”, may be formed in a polymeric security document which hasopacifying layers on both sides by omitting the opacifying layers on oneside only of the security document in the window area so that the“half-window” is not fully transparent, but allows some light to passthrough without allowing objects to be viewed clearly through thehalf-window.

Alternatively, it is possible for the substrates to be formed from ansubstantially opaque material, such as paper or fibrous material, withan insert of transparent plastics material inserted into a cut-out, orrecess in the paper or fibrous substrate to form a transparent window ora translucent half-window area.

Opacifying Layers

One or more opacifying layers may be applied to a transparent substrateto increase the opacity of the security document. An opacifying layer issuch that L_(T)<L₀, where L₀ is the amount of light incident on thedocument, and L_(T) is the amount of light transmitted through thedocument. An opacifying layer may comprise any one or more of a varietyof opacifying coatings. For example, the opacifying coatings maycomprise a pigment, such as titanium dioxide, dispersed within a binderor carrier of heat-activated cross-linkable polymeric material.Alternatively, a substrate of transparent plastic material could besandwiched between opacifying layers of paper or other partially orsubstantially opaque material to which indicia may be subsequentlyprinted or otherwise applied.

Refractive Index

The refractive index of a medium n is the ratio of the speed of light invacuum to the speed of light in the medium.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings. It is to be appreciated that the embodiments aregiven by way of illustration only and the invention is not limited bythis illustration. In the drawings:

FIG. 1 shows a security document including a security device having avolume hologram and an optional security feature;

FIG. 2a shows a substrate;

FIG. 2b shows the substrate including an optically sensitive material(OSM) applied to a region of a first surface of the substrate;

FIG. 2c shows the security device located within a half-window region ofthe security document;

FIG. 2d shows the security device located within a window region of thesecurity document;

FIG. 2e shows the security device located within a window region and anoptional security feature located within a half-window region;

FIG. 3 shows a reference beam and an imaging beam incident onto oppositesides of the OSM;

FIG. 4 shows an apparatus for recording a volume hologram;

FIG. 5a shows the OSM with an outward facing surface shaped as anarrangement of microlenses; and

FIG. 5b shows the OSM with an outward facing surface shaped as adiffraction grating.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a security document 2 including anoptical device 4, preferably providing the functionality of a securitydevice, and one or more optional security features 6. The optionalsecurity features 6 can be, for example, further optical-based devicessuch as optically variable devices. The optical device 4 can be locatedwithin a window or half-window region of the security document 2. Theoptical device 4 is a volume hologram providing a visual effect.

Referring now to FIGS. 2a to 2e , a method for producing a securitydocument 2 including an optical device 4 corresponding to a volumehologram is shown. Initially, there is provided a substrate 8 (FIG. 2a). The substrate 8 can be a polymer substrate, for example based onbiaxially oriented polypropylene (BOPP). Such a substrate 8 is typicallytransparent or translucent. An optically sensitive material (OSM) 10 isdirectly applied to a region of a surface of the substrate as shown inFIG. 2b (the optional protective coating 15 is described below).

In an embodiment, the OSM 10 is applied using printing and/or embossingtechniques. However, other printing methods suitable for directlyapplying the OSM 10 onto the region of the surface of the substrate 8may be utilised. The OSM 10 is a printable photopolymer suitable forrecording a volume hologram.

One form of printing which allows for accurate registration is Simultanprinting. Other suitable forms of printing/embossing include gravureprinting and intaglio printing. These printing/embossing techniques alsoallow for accurate registration.

Registration between the OSM 10 and the substrate 8 can beneficiallyprovide a means for ensuring that the OSM 10 is applied in substantiallythe same region of each substrate 8 of a plurality of substrates in acontinuous printing process. Printing/embossing techniques also allowfor accurate registration with a region corresponding to a window orhalf-window.

FIGS. 2c to 2e show different embodiments of the security document 2including the OSM 10, a first opacifying layer 12, and a secondopacifying layer 14. The opacifying layers 12, 14 can be applied usingknown printing techniques such as gravure printing, and can be appliedto form half-window or full window areas in registration with the OSM10.

FIG. 2c corresponds to an embodiment with the second opacifying layer 14applied to the substrate 8 in the region where the optical device 4 islocated so that the optical device 4 is located within a half-windowregion 16 of the security document 2. In this embodiment, the volumehologram will be visible from one side of the security document 2 only.

FIG. 2d corresponds to an embodiment with the second opacifying layer 14omitted in the region where the optical device 4 is located so that theoptical device 4 is located within a full window region 18 of thesecurity document 2. In this embodiment, the volume hologram can bevisible from one side or both sides of the security document 2,depending on the configuration of the volume hologram.

It is also possible for another security feature, such as another volumehologram or other optically variable feature, to be provided on theopposite of the substrate 8 from the OSM 10, either in the half-window16 of FIG. 2c or within the full window region 18 of FIG. 2 d.

FIG. 2e corresponds to an embodiment wherein the optical device 4 islocated within a window region 18 of the security document 2, and anoptional security feature 6 is located within a half-window region 20(though it is understood that the optional security feature 6 could belocated in a window region). The half-window region 20 (or,equivalently, the window region) of the optional security feature 6 canbe contiguous or separate to the window 18 or half-window 16 region ofthe optical device 4.

Though two opacifying layers 12, 14 are shown in each of FIGS. 2c to 2e, it is envisaged that only one opacifying layer 12 or 14 may beapplied. It is also possible for more than one opacifying layer to beprovided on one or both sides of the substrate 8, and/or for thethicknesses of the opacifying layers 12, 14 to be varied, e.g. to createshadow images. Furthermore, though the first opacifying layer 14 isshown in contact with the OSM 10, it may be that the OSM 10 is locatedwithin, and not bordering, the first opacifying layer 12. Opacifyinglayers 12, 14 allow for printing, for example of images and/or patterns,on the substrate 8. Opacifying layers 12, 14 can be a single colour, forexample white, suitable for subsequent printing of images and/orpatterns and/or text etc. Registration can allow for precise and/orconsistent relative positioning between the OSM 10 and any images and/orpatterns, such that each security document 2 produced is identical or atleast substantially identical.

The OSM 10 can be applied such that the average and/or total thicknessof the OSM 10 is not more than about 20 microns, preferably not morethan about 10 microns. It can be advantageous to provide a suitably thinOSM 10 such that the optical device 4 formed by the OSM 10 is suitablefor use in a print feed. In order to be able to record a volume hologramwithin the OSM 10, there exists a minimum thickness suitable for the OSM10, which can be determined, for example, through experiment and/orcalculation. Preferably, the minimum thickness is such that theresulting hologram is not a “thin” hologram. The OSM 10 can be appliedin the form of a patch or stripe. Alternatively, the OSM 10 can beapplied such that a complex shape with varying shape and/or size isdefined. In one example, the OSM 10 is printed such that it definesinternal areas not containing a volume hologram.

In an embodiment, in order to assist with adhesion of the OSM 10, thesubstrate 8 includes a primer layer, at least in the region of the OSM.The primer layer preferably includes a polyethylene imine. The primerlayer may also include a cross-linker, for example a multi-functionalisocyanate. Examples of other primers suitable for use in the inventioninclude: hydroxyl terminated polymers; hydroxyl terminated polyesterbased co-polymers; cross-linked or uncross-linked hydroxylatedacrylates; polyurethanes; and UV curing anionic or cationic acrylates.Examples of suitable cross-linkers include: isocyanates; polyaziridines;zirconium complexes; aluminium acetylacetone; melamines; andcarbodi-imides. The primer layer may itself include multiple layers.

Before the opacifying layers 12, 14 are applied to the substrate 8, theOSM 10 is exposed to patterned radiation, preferably sourced from alaser. Alternatively, the OSM 10 can be exposed to the patternedradiation after application of the opacifying layers 12, 14, which maybe particularly applicable when the OSM 10 is located within a windowregion of the security document 2. The OSM 10 is initially responsive toincident light, wherein the OSM 10 can be sensitive to a wide-range ofwavelengths, or a specified range of wavelengths. Said wavelengths canbe, for example, in one or more of the visible spectrum, infra-redspectrum, and ultra-violet spectrum. The OSM 10 is configured forrecording a pattern of radiation intensity, through local changes inrefractive index in proportion to local radiation intensity.

The patterned radiation may be provided using known volume holographicpreparation techniques. In general, a reference beam 26 and an imagingbeam 28 are directed such as to create an interference pattern withinthe OSM 10 (the inference pattern corresponds to the patternedradiation). This can be achieved, with reference to FIGS. 3 and 4, bydirecting the reference beam 26 onto a first surface 30 of the OSM 10,and the imaging beam 28 onto a second surface 32 of the OSM 10, whereineither the reference beam 26 or the imaging beam 28 is directed throughthe substrate 8, thereby creating a reflection-type volume hologram. Thereference beam 26 corresponds to coherent radiation directly sourcedfrom the laser. The imaging beam 28 corresponds to patterned radiation.The patterned radiation of the imaging beam 28 is, usually, sourced fromthe same coherent source of radiation 40 as the reference beam, but isfirst either reflected or transmitted through a master volume hologram,or alternatively scattered from an object, before being incident ontothe second surface 32, as shown in the apparatus of FIG. 4. Theapparatus may also include a beam splitter 44 and one or more mirrors 46for directing the reference beam 26 and/or imaging beam 28 onto the OSM10.

In an alternative arrangement, both the reference beam 26 and imagingbeam 28 are directed onto the same side of the OSM 10, creating atransillumination-type volume hologram. Embodiments having the opticaldevice 4 located in a full window (such as shown in FIG. 2d ) may beparticularly suitable for this arrangement.

A master volume hologram 42 will produce an imaging beam 28corresponding to an original imaging beam used to create the mastervolume hologram 42, on transmission or reflection of incident coherentradiation (i.e. a reference beam). Therefore, a master volume hologram42 creates the same imaging beam 28 as scattering off an object, wherethe object is the same object as was used to create the master volumehologram 42.

One exemplary technique for recording a volume hologram into the OSM 10is to direct an incident reference beam 26 through the first surface 28of the OSM 10, and providing a reflective master volume hologram 32adjacent the second surface 30 of the OSM 10 (either directly adjacentor including a spacing). When the reference beam 26 is reflected fromthe master volume hologram 26, it is effectively modulated into theimaging beam 28, and is directed onto the second surface 28 due to thereflection. This imaging beam 28 is then able to create the appropriateinterference pattern within the OSM 10 due to interference with thereference beam 26.

Volume holograms can be angularly and wavelength dependent. Therefore,multiple images can be recorded within the OSM 10, for example bysimultaneously or sequentially irradiating the OSM 10 with laser lightof different wavelength and/or with different incident angles for thereference beam 26.

The OSM 10 can be desensitised during the exposure to the patternedlight, such that no further change in the optical properties of the OSM10 occurs after the volume hologram has been recorded. However, ableaching step can be included after exposure to the patterned light inorder to improve the optical transparency of the OSM 10. The bleachingstep can correspond to irradiation with UV light.

Optionally, a protective coating 15 is applied to the OSM 10, before orafter recoding the interference pattern and/or desensitising the OSM 10.This is particularly useful when the OSM material is not sufficientlydurable for the intended task (for example, for extensive public usewhen used with banknotes). The protective coating 15 should betransparent or at least substantially transparent, such that the qualityof the volume hologram is not noticeably diminished by the presence ofthe protective coating 15. Alternatively, the protective coating 15 istransparent only to a selection of wavelengths, thereby acting as afilter.

In order to increase the visibility of the volume hologram opticaleffect, the OSM 10 can be positioned adjacent a dark, preferably black,background. Such background can be created by applying a printed patchto the substrate 8.

According to an embodiment, the OSM 10 has a non-flat outward facingsurface. Such an arrangement can provide for interesting further opticaleffects in addition to the volume hologram recorded into the OSM 10. Inone implementation, the OSM 10 is applied using an embossing process andthe embossing tool includes a non-planar profile, the reverse of whichit imparts onto the OSM 10.

FIG. 5a shows the OSM 10 applied to the substrate 8 and including anoutward facing surface 16 a shaped as to form an array of microlenses.The OSM 10 in this case has a sufficiently different refractive index toair (as shown) or a protective coating applied to the OSM 10 (not shown)required to provide a refractive lens effect. The OSM 10 therebyprovides both a volume hologram optical effect (the volume hologram isrecorded as described herein) and a microlens array effect. Suchmicrolens arrays can be utilised in a number of known ways in order toprovide additional security to the security document 2, for example aspart of a moiré optical device, a contrast switch, or an integralimaging device. The OSM 10 may be positioned overlapping a printedpattern configured to interact with the microlenses in order to providethe additional optical effect. Suitable printed patterns includearrangements of microimages.

FIG. 5b shows the OSM 10 applied to the substrate 8 and including anoutward facing surface 16 b shaped as to form a diffraction grating,which may be a thin hologram. Similar to FIG. 5a , the OSM 10 has asufficiently different refractive index to air (as shown) or aprotective coating applied to the OSM 10 (not shown) required to providea diffractive effect.

Further modifications and improvements may be made without departingfrom the scope of the present invention. For example, the master volumehologram may be replaced with a diffractive-based optical device, forexample a Kinegram®, a simple blazed diffraction grating, etc. Anotherembodiment may utilise the known technique of double soft-embossing toemboss, in register, optically sensitive materials on each side of thesubstrate.

1.-30. (canceled)
 31. A method for producing a security documentcomprising a volume hologram, including the steps of: a) applying anoptically sensitive material onto a first surface of a substrate in afirst region of the first surface; and b) irradiating the opticallysensitive material with patterned radiation configured for recording avolume hologram within the optically sensitive material.
 32. A method asclaimed in claim 31, where the optically sensitive material is appliedusing a printing and/or embossing process.
 33. A method as claimed inclaim 31, including the further step of applying a protective coating toan outwards facing surface of the optically sensitive material.
 34. Amethod as claimed in claim 33, wherein the protective coating is appliedafter the irradiation of the optically sensitive material with patternedlight.
 35. A method as claimed in claim 31, wherein the substrate isformed from a polymeric material, preferably biaxially orientedpolypropylene.
 36. A method as claimed in claim 31, wherein theoptically sensitive material is a printable photopolymer.
 37. A methodas claimed in claim 31, wherein once applied, the optically sensitivematerial includes a substantially flat outward facing surface.
 38. Amethod as claimed in claim 31, wherein once applied, the opticallysensitive material includes a non-flat outward facing surface, such thatthe optical sensitive material provides a further optical effect inaddition to providing a volume hologram.
 39. A method as claimed inclaim 38, wherein the outward facing surface is shaped as an arrangementof microlenses or is shaped as a diffraction grating.
 40. A method asclaimed in claim 31, wherein the thickness of the optically sensitivematerial is not greater than 20 microns, preferably not greater than 10microns.
 41. A method as claimed in claim 40, wherein the minimumthickness of the optically sensitive material is sufficient to record avolume hologram.
 42. A method as claimed in claim 31, including the stepof: applying a first opacifying layer to the first surface, the firstopacifying layer being omitted in a first window region, wherein theoptically sensitive material is located within the first window region.43. A method as claimed in claim 42, wherein the first opacifying layerand the optically sensitive material are applied in registration, suchthat the first window region is registered with the optically sensitivematerial.
 44. A method as claimed in claim 42, including the step ofapplying a second opacifying layer to a second surface of the substrate.45. A method as claimed in claim 44, wherein the second opacifying layeris configured to partially or entirely cover the optically sensitivematerial, such that where the second opacifying layer covers theoptically sensitive material, the volume hologram is only visible fromthe first surface.
 46. A method as claimed in claim 44, wherein thesecond opacifying layer is omitted in a second window region, whereinthe optically sensitive material is visible through the second windowregion of the second opacifying layer.
 47. A method as claimed in claim42, wherein the, or each, opacifying layer is applied after theirradiation with patterned radiation.
 48. A method as claimed in claim31, wherein the substrate includes a primer layer, such that theoptically sensitive material is applied directly to the primer layer.49. A method as claimed in claim 48, wherein the primer includes apolyethylene imine.
 50. A method as claimed in claim 31, whereinpatterned radiation is provided by one or more lasers.
 51. A method asclaimed in claim 50, wherein the patterned radiation is created at leastin part through reflection of laser radiation of an object.
 52. A methodas claimed in claim 50, wherein the patterned radiation is created atleast in part through the provision of a master hologram.
 53. A methodas claimed in claim 31, wherein the patterned radiation includeswavelengths selected from one or more of the visible spectrum, infra-redspectrum, and ultra-violet spectrum.
 54. A security document including asubstrate having a first opacifying layer applied to a first surface,the first opacifying layer including a first window region in which avolume hologram is located, said volume hologram formed from anoptically sensitive material applied to the first surface.
 55. Asecurity document as claimed in claim 54, wherein the substrate includesa primer layer, and wherein the optically sensitive material is applieddirectly to the primer layer.
 56. A security document as claimed inclaim 54, wherein the substrate has a second opacifying layer applied toa second side.
 57. A security document as claimed in claim 56, whereinthe second layer partially or entirely covers the volume hologram.
 58. Asecurity document as claimed in claim 66, wherein the second layerincludes a second window region substantially in register with the firstwindow region in which the volume hologram is located, such that thevolume hologram is located in a full window of the security document.59. A security document produced by the method of claim
 31. 60. Asecurity document as claimed in claim 54, wherein the security documentis a banknote.